Originally Posted by
Six jours
If a baseball bat was capable of accelerating a person's entire body, you'd have a point.
At any rate, your stuff is getting so silly that it's becoming apparent that you are emotionally invested in the idea of your helmet saving you if you are struck by a car. Your powers of self-delusion are obviously superior to my powers of persuasion, so this line of discussion is looking fairly pointless.
Seriously the same physical principles are at play with a baseball bat or car. The only difference is that the bat has greater change in momentum, so difference in acceleration is less pronounced. But there is a difference in accelerations there no matter what.
I'll explain it explicitly. Acceleration is the change in velocity over the amount of
time during which the acceleration is experienced. The body may deform when struck, and may (or sections of it may) also rotate when struck. When the body deforms, the momentum transfers over a larger period of time, reducing the acceleration. If the body rotates, the head describes an arc while following, which also increases the time under acceleration, which decreases the acceleration experienced.
I'm not trying to persuade you, and I'm am subject to persuasion through an explanation of the relevant science or credible data. I merely see a misunderstanding of the physical dynamics and thought that it would be a shame if you, or anyone else, refrained from wearing a helmet on a particular ride solely because they mistakenly believed that it would necessarily be of limited or no use.